A liquid crystal display (LCD) device includes an LCD panel and a backlight module. The backlight module includes a light emitting diode (LED) backlight driving circuit. As shown in FIG. 1, the LED backlight driving circuit includes an LED lightbar 30, a power module 10 driving the LED lightbar 30, and a constant current driving chip 20 that controls the power module 10. The constant current driving chip 20 receives an external pulse-width modulation (PWM) dimming signal to control an effective current flowing through the LED lightbar 30, thus, influencing brightness of the LED lightbar 30.
The constant current driving chip 20 outputs a driving signal to a metal-oxide-semiconductor field-effect transistor (MOSFET) of the power module 10. When the MOSFET turns on, an inductor stores energy. When the MOSFET turns off, the inductor releases energy to provide a high voltage for the LED lightbar. An equation of an output voltage is: V0=Vin/(1-D), where D is a duty cycle of a driving waveform. Power loss of the MOSFET is in connection with a frequency of the driving signal and a turn-on time in a unit period. When the frequency of the driving signal is great, a switching loss of the MOSFET is great, but turn-on time in unit period is short, and a conduction loss of the MOSFET is small. When the frequency of the driving signal is small, the switching loss of the MOSFET is small, but turn-on time in unit period is long, and the conduction loss of the MOSFET is great.